Exploring the Stability Landscape of Constant-Stress Pumpkin Balloon Designs

Abstract

Pumpkin balloon designs such as the constant-bulge-angle design, the constant-bulge-radius design, and hybrids of the constant-bulge-angle and constant-bulge-radius schemes have been used in an attempt to achieve a cyclically symmetric pumpkinlike shape when fully inflated. A number of flight balloons that were built based on the constant-bulge-angle, constant-bulge-radius, and hybrid design strategies encountered deployment problems. In June 2006, Flight 555-NT (a hybrid design) formed an S-cleft and did not deploy. Currently, NASA's approach to superpressure balloon design uses a constant-stress model developed at NASA Goddard Space Flight Center. To fully understand the mechanism behind cleft formation in pumpkin balloons and to explore the constant-stress design space, NASA's Balloon Program Office carried out a series of inflation tests in 2007 involving four 27-meter-diameter 200-gore pumpkin balloons. One of the test vehicles was a one-third-scale mockup of the Flight 555-NT balloon. Using an inflation procedure intended to mimic ascent, the one-third-scale mockup developed an S-cleft feature that was strikingly similar to the one observed in Flight 555-NT. The remaining three 27-meter balloons tested were constant-stress designs and deployed properly. In an effort to gauge constant-stress design susceptibility to deployment problems, we carry out a number of parametric studies and assess the stability landscape of the constant-stress design space. In our studies, we examine two types of top end-fitting boundary conditions, one restrictive and one less restrictive, that help to define a cleft-free design envelope. We correlate our analytical predictions with outcomes of inflation tests involving 27-meter-diameter test vehicles and outcomes from flight 586-NT and flight 591-NT, which involved larger constant-stress designs. To study scaling effects, we consider a 14-million-cubic-foot design. Our analysis suggests that as one scales up the balloon, the size of the cleft-free envelope shrinks.